1. Field of the Invention
The present invention relates to an optical transmission system that reduces residual dispersion and enables high bit-rate transmissions, in particular to methods employing dispersion compensating fibers.
2. Description of the Related Art
Due to the practical application of erbium-doped optical fiber amplifiers, ultra-long distance, non-regenerating relays and other optical transmission systems using optical amplifiers have already been commercialized in the C-band. In addition, accompanying increases in communication capacity, the development of wavelength division multiplexing (WDM) transmission is proceeding at a rapid pace, and systems have already been commercialized for several optical transmission paths. In the future, although even greater wavelength multiplexing is expected to increase by using broader band and reducing the wavelength interval, the transmission speed per wavelength is also expected to increase.
In order to employ WDM transmission system, it is important that the gain difference caused by the erbium-doped optical fiber amplifier, including the transmission loss of the optical fiber for transmission, be minimized at the wavelength band used, and that the wavelength dispersion be somewhat large at an intermediate point in transmission, and be somewhat small over the entire transmission band with respect to the overall transmission system.
In addition, in recent long-distance systems, the number of multiplexed wavelengths has increased rapidly, and due to the rapid increase in the optical power propagating through the optical fibers, technology for suppression of nonlinearity is essential. The magnitude of this nonlinearity is represented with the following formula:
 n2/Aeff
where, n2 represents the non-linear refractive index of the optical fiber, and Aeff represents the effective area of the optical fiber. Although it is necessary to either reduce n2 or increase Aeff in order to reduce nonlinearity, since n2 is value that is characteristic to the material, it is difficult to lower this value significantly in quartz-based optical fibers. Consequently, the emphasis of current development of suppression of nonlinearity is focused on increasing Aeff.
At present, 1.3 μm band optimized, single-mode optical fiber networks are spread throughout the world. When transmission at 1.55 μm is carried out using this optical fiber network, wavelength dispersion of about +17 ps/nm/km occurs. Consequently, when signals are transmitted using this optical fiber, transmission characteristics deteriorate considerably due to the effects of wavelength dispersion.
Consequently, development has proceeded on dispersion compensating fibers in order to compensate for this wavelength dispersion, and optical fibers of this type have already been commercialized. This dispersion compensating fiber has considerable negative dispersion at operating wavelength band, and by connecting suitable length of dispersion compensating fiber and optical fiber for transmission, the positive dispersion generated with single-mode optical fiber for transmission can be compensated. Although the residual dispersion impairs high-speed transmission, high-speed communication is possible by compensating for accumulated dispersion in this manner.
The demand for dispersion compensating fiber modules that compensate the dispersion slope and wavelength dispersion of optical transmission paths using 1.3 μm band optimized, single-mode optical fibers is growing rapidly. These modules can be manufactured by technology such as that of the following inventions previously filed by the authors and in academic reports and so forth.
Examples of patent applications include Japanese Unexamined Patent Application, First Publication Nos. 2001-318259, 2001-337245, 2002-98853, 2002-55251, and 2002-71996, while embodiments of academic publications include “Large-effective-area dispersion compensating fibers for dispersion accommodation both in the C and L band”, OECC' 00, Technical Digest, 14C4-4, pp. 554-555, 2000.
The form of the refractive profile of dispersion compensating fibers is shown in FIG. 3, and by suitably setting the outer diameter ratio and refractive index of each layer, low nonlinearity can be maintained while increasing effective area.
By using these technologies, the accumulated dispersion of a transmission path can be compensated over a wide wavelength range as shown in FIG. 4.
However, since transmission speed is restricted by the accumulated dispersion of the optical transmission path, there is a growing demand for dispersion-slope and dispersion compensating fiber modules capable of compensating dispersion over a wide wavelength band instead of a single wavelength of the operating wavelength band for use as dispersion compensating fiber modules. Here, the dispersion slope compensation ratio is the ratio of the dispersion slope of the dispersion compensating fiber to the dispersion slope of the single-mode optical fiber for transmission divided by the ratio of the dispersion value of the dispersion compensating fiber to the dispersion value of the single-mode optical fiber for transmission. The relationship between accumulated dispersion and transmission speed is shown in FIG. 5.
Although transmission distance is restricted by the polarization dispersion and transmission loss of the transmission path, the transmission distance of a 1.3 μm band optimized, single-mode optical fiber is roughly 400 km in consideration of the loss of the dispersion compensating fiber module and so forth. According to FIG. 5, in order to perform high-speed transmission at 40 Gb/s, it is necessary that residual dispersion be ±65 ps/nm or less. In order to achieve this high-speed transmission, the dispersion of the dispersion compensating fiber module is required be accurately matched to the residual dispersion that has accumulated in the transmission path so as to be able to compensate that dispersion, and the dispersion slope compensation ratio must also be nearly 100%.
Although the dispersion-slope and dispersion compensating fibers used in dispersion compensating fiber modules have been able to obtain a dispersion slope compensation ratio of nearly 100% due to advances in refractive index profile control technology, due to the extremely high sensitivity of the dispersion slope compensation ratio relative to changes in the refractive index profile, these optical fibers have a certain degree of dispersion slope compensation ratio when mass produced.
For example, in order for a 1.3 μm band optimized, single-mode optical fiber that is the target of dispersion compensation to be compensated for dispersion in a wavelength band of 1530-1570 nm, although the dispersion error can be held to within ±1.0% by finely adjusting the required amount of dispersion at 1550 nm, which is the central wavelength of this wavelength band, and the dispersion slope compensation ratio can be made to be 100±10% by selecting a dispersion compensating fiber, when a module is attempted to be manufactured using such a high-performance dispersion compensating fiber, it causes a decrease in the yield of optical fiber and a corresponding increase in costs.
In addition, even if such a high-performance dispersion compensating fiber module is used, if there is bias in the dispersion slope compensation ratio of this dispersion compensating fiber within a span of about 400 km, residual dispersion ends up exceeding ±65 ps/nm at the wavelengths on both ends of the operating wavelength range. For example, FIG. 6 depicts the residual dispersion following compensation when wavelength dispersion of a transmission path having a transmission distance of about 400 km was compensated about every 80 km using a dispersion compensating fiber having a dispersion error at the central wavelength of 0.7% and dispersion slope compensation ratio of 100±10%.
As can be understood from FIG. 6, the maximum value of residual dispersion following compensation was ±95 ps/nm at 1530 nm and 1570 nm corresponding to the wavelengths at both ends of the operating wavelength range. Consequently, in the case of high-speed transmission of 40 Gb/s or more, the transmission characteristics of the optical transmission system deteriorate according to the characteristics and arrangement of the dispersion compensating fiber.